Max out the RAM and upgrade your internal drive to an SSD. The library files for iTunes reside on your boot drive and no matter what drive the actual music/video files are on, any RPM of hard drive is more than fast enough to stream audio and video in the bandwidths used by MPEG4/H.264 formats, even HD stuff and audio is even less demanding whether it's AAC/MP3, lossless or even raw audio like a stereo WAV or AIFF.